Image reconstruction method

ABSTRACT

A method of extracting the shape of a probe tip of a probe-based instrument from data obtained by the instrument is provided. The method employs algorithms based on the principle that no reconstructed image points can physically occupy the same region as the tip during imaging. Sequential translates of the tip shape or volume sweep out an area or volume that is an “exclusion zone” similar to morphological erosion. The embodiments of the alternative method use either the region defined by the tip boundary or simply the tip boundary.

RELATED PATENT APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/944,333 filed Sep. 17, 2004, now U.S. Pat. No. 7,143,005, whichis a continuation-in-part (CIP) of U.S. patent application Ser. No.10/139,949 filed May 6, 2002, now U.S. Pat. No. 6,810,354, and herebyincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to scanning probe microscopes (SPMs),and more particularly, to alternate methods of accounting for the shapeof the probe tip in the acquired image, the methods being particularlyadapted for measuring critical dimensions (CD) and features ofsemiconductor wafers, data recording media, and related.

2. Description of Related Art

Several known probe-based instruments monitor the interaction between acantilever-based probe and a sample to obtain information concerning oneor more characteristics of the sample. For example, SPMs, such as theatomic force microscope (AFM), are devices which typically use a sharptip and low forces to characterize the surface of a sample down toatomic dimensions. More particularly, SPMs monitor the interactionbetween the sample and the tip (where the tip is typically mounted onthe cantilever of the probe). By providing relative scanning movementbetween the tip and the sample, surface characteristic data can beacquired over a particular region of the sample, and a corresponding mapof the sample can be generated.

The atomic force microscope (AFM) is a very popular type of SPM. Theprobe of the typical AFM includes a very small cantilever which is fixedto a support at its base and which has a sharp probe tip extending fromthe opposite, free end. The probe tip is brought very near to or intocontact with a surface of a sample to be examined, and the deflection ofthe cantilever in response to the probe tip's interaction with thesample is measured with an extremely sensitive deflection detector,often an optical lever system such as described in Hansma et al. U.S.Pat. No. RE 34,489, or some other deflection detector such as straingauges, capacitance sensors, etc. The probe is scanned over a surfaceusing a high resolution three axis scanner acting on the sample supportand/or the probe. The instrument is thus capable of creating relativemotion between the probe and the sample while measuring the topographyor some other surface property of the sample as described, e.g., inHansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No.5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.

AFMs may be designed to operate in a variety of modes, including contactmode and oscillating mode. In contact mode operation, the microscopetypically scans the tip across the surface of the sample while keepingthe force of the tip on the surface of the sample generally constant.This effect is accomplished by moving either the sample or the probeassembly vertically to the surface of the sample in response to senseddeflection of the cantilever as the probe is scanned horizontally acrossthe surface. In this way, the data associated with this vertical motioncan be stored and then used to construct an image of the sample surfacecorresponding to the sample characteristic being measured, e.g., surfacetopography. Alternatively, some AFMs can at least selectively operate inan oscillation mode of operation such as TappingMode™. (TappingMode™ isa trademark of the present assignee.) In oscillation mode, the tip isoscillated at or near a resonant frequency of the cantilever of theprobe. The amplitude or phase of this oscillation is kept constantduring scanning using feedback signals, which are generated in responseto tip-sample interaction. As in contact mode, these feedback signalsare then collected, stored, and used as data to characterize the sample.

Regardless of their mode of operation, AFMs can obtain resolution downto the atomic level on a wide variety of insulating or conductivesurfaces in air, liquid or vacuum by using piezoelectric scanners,optical lever deflection detectors, and very small cantileversfabricated using photolithographic techniques. Because of theirresolution and versatility, AFMs are important measurement devices inmany diverse fields ranging from semiconductor manufacturing tobiological research.

Notwithstanding the fact that scanning probe microscopes are highresolution instruments, the ultimate resolution of the data obtained bysuch probe-based instruments is limited by the physical characteristicsof the tip of the probe itself. More particularly, there are limitationsas to how small, and sharp, the tip can be made. In view of this, thetip shape is reflected in the acquired data, a problem that isexacerbated by the fact that AFMs often image very small (e.g., Angstromscale) features. As a result, an error in the acquired data results andthe corresponding accuracy of the surface image is significantlycompromised. Hereinafter, the acquired SPM image will periodically becalled the “dilated” image.

For some applications, this limitation may be negligible. However, formany applications, the degree of accuracy required to resolve thefeatures of the sample surface is significantly greater, such that tipshape error is unacceptable. For instance, in the semiconductorfabrication industry, imaging features such as lines, trenches and viaswith single nanometer accuracy is desired. These features may havedimensions in the range of about 90 nm, and are continually gettingsmaller. With typical tip widths in the range of about 70 nm, the tipshape clearly introduces significant error in the data and must beremoved to accurately image the sample surface.

Moreover, the aforementioned problems can be exacerbated by the factthat complex sample surface topologies require a commensurate increasein tip shape complexity to image such surfaces. For example, samples mayinclude undercut regions where a particular x,y scan position may havemultiple “Z” height values (see region “U” in FIG. 1 , discussed infurther detail below). Again, this is common in the semiconductorfabrication industry, and thus tips have been developed to allow imagingof such complex topographies. However, with the increase in tip shapecomplexity, there typically is a corresponding increase in error in theAFM data.

Two types of known tip shapes are illustrated in FIGS. 1 and 2. Notethat probe tips, such as the CD tip, shown in FIG. 2, typically will nothave the smooth symmetrical shapes illustrated in the figures. These tipshapes are merely presented as such to highlight the concepts andfeatures of the preferred embodiment. In FIG. 1, a probe tip 10 of atraditional scanning probe microscope includes a parabolic, or otherpointed shape that is relatively easy to characterize. Tip 10 includes ashaft 12 and a distal end 14 that although sharp is typically at leastslightly rounded at its active surface 15. During a scan (operating inan oscillating mode, for instance), tip 10 interacts with a samplesurface 16 to image characteristics of that surface. Tip-sampleinteraction is controlled, and data is collected, via a control system(not shown) as described previously. The collected data, in turn, may beplotted to image the sample surface. Importantly, this acquired imagemay not accurately reflect sample surface characteristics due to, amongother things, the error introduced by the shape of the pointed tip.

In addition to introducing at least some tip shape error in the acquireddata, probe tip 10 is unable to image certain surfaces. In particular,although suitable for many applications, based on its shape probe tip 10is simply unable to accurately depict vertical sidewalls and undercutregions (which often exist in semiconductor fabrication, for example) inthe corresponding sample surface topography. Notably, this is due tolimitations in both the tip shape and the algorithms used to control tipposition.

To be able to image surface features such as vertical sidewalls andundercut regions, AFMs having more complex probe tips have beendeveloped. In one such instrument, shown in FIG. 2, an AFM employsactive X-Z control to follow complex surface topography using a probetip 20 having a shaft 22 and a distal end 24 including left and rightprotuberances 26, 28, respectively, in the scan (for example X)direction. By dithering the tip in the scan direction, protuberances 26,28 are caused to interact with surface features such as verticalsidewalls. As a result, what before caused “shaded regions” (i.e.,regions of no tip-sample contact such as undercut region “U” illustratedin FIGS. 1 and 2) in the acquired AFM data, now yields at least somedata based on tip-sample contact. However, with this increase inflexibility of the types of samples that can be imaged, correcting andreconstructing the image data becomes increasingly difficult.

Overall, whether employing simple or complex probe tip shapes, theproblem of the shape of the tip being convolved in the AFM data has beenknown and appreciated in the art. Although solutions have been attemptedwith some success for conventional AFMs, extracting tip shape errorsfrom CD AFM data has been an inexact process. Moreover, as featuresbecome smaller, and because the tip is at least somewhat limited in justhow small it can be made, the convolution of the tip in the image databecomes more substantial, and thus it is becoming increasingly importantthat the tip shape be removed for accurate measurements.

In another known and widely used technique, particularly applicable tothe above-described CD probe shown in FIG. 2, rather than applying shape“deconvolution” of the image to compensate for the effect of dilation ofthe image, a simple subtraction of the tip-width in the scan directioncan provide improved reconstructed images and critical dimensionmeasurements.

For this technique to provide a useful correction, the width of the CDtip must be determined to a high degree of accuracy. The way in whichthis is typically accomplished is by scanning a silicon nanoedge with,for example, the boot shaped CD tip shown in FIG. 2. Because thedimensions of the nanoedge are known or at least very closelyapproximated, the width of the tip can be extracted from the image data.This scan of a silicon nanoedge is illustrated in FIG. 3A. Inparticular, a CD tip (for example, 20 in FIG. 2) is scanned from left toright over an improved silicon nanoedge (ISNE) 31 so as to produce animage data profile 30. In this method, the width of the tip iscalculated according to,W _(tip) =L−(W ₁ +W ₂)  Equation 1where “L” is the total width of the acquired image a vertical distance“D” (defined below) from the plateau. W₁ and W₂ are defined as follows,W ₁=(D−r)tan α+r  Equation 2andW ₂=(D−r)tan β+r.  Equation 3

In these equations, “D” is the distance from the plateau “P” of thescanned image used for measuring the angles α and β, as illustrated inFIG. 3. For example, this value may be approximately 800 angstroms. Inaddition, “r” is the radius of the vertex of the ISNE, estimated by SEM,TEM and/or sharp tip SPM analysis of the nanoedge, and is approximately75 angstroms. The angles α and β are the angles computed from the leftand right side slopes, respectively, of the previous tip calibrationanalysis. Computing the tip width in this fashion, this prior art methodcan be used to subtract off that width from the image data generatedduring a scan to approximately correct for the error in the image data.Although providing a correction, this method has significant drawbacks.

First, by simply subtracting the tip width from the image data, it isassumed that the tip-sample contact is being made at a particular point,for example, at the vertical tangent of the protuberances of the bootshaped or CD tip (i.e., at point 29, FIG. 2). However, as the tip scansalong a particular topography, the contact point of the tip on thesample translates along the surface of the tip and thus the effectivewidth of the tip at the contact point changes. As a result, asingle-valued tip width subtraction is inexact. By simply subtractingoff a single value tip-width, an error remains in the reconstructedimage as each feature of a unique tip shape cannot be fully accountedfor in correcting AFM image data. These errors are directed toinaccuracies in the image of the sample surface shape for both topologyand CD width measurements at a particular height. Another significantdrawback is that the width defined in Equations 2 and 3 set forth aboveare merely estimates for the actual tip width. As the samples to beimaged continue to demand greater resolution, these equations willbecome inadequate even for those applications where tip-width correctionprovides an acceptable correction.

In short, for the applications contemplated by the present invention, noknown technique sufficiently accounts for the tip shape whenreconstructing CD AFM image data.

For reconstructing non-reentrant, relatively simple topologies, themethods using local slope-matching between the acquired image data andthe tip profile have been attempted. A drawback of slope-matching, or“slope-based,” reconstruction methods is that Legendre transforms usedin the analysis, which require numerical derivatives of the data, can behighly sensitive to noise in the original image data. A “smoothing”technique may be implemented to reduce the noise enough to allowreconstruction, but such smoothing typically eliminates sharp features,which are often critical to accurate reconstruction.

A known method to reduce the negative effects of noise is use of amedian filter in pre-processing image data prior to slope-basedreconstruction of the image data. However, due to inherent limitations,the use of median filters alone does not solve the problem of noiseamplification and artifact generation in the reconstructed image.Certain known techniques to smooth or reduce noise can also eliminatecrucial features in an image, thereby causing false imagereconstruction.

In view of the above drawbacks with known methods of smoothing andpre-filtering original image data, an improved method is desired toreduce noise and artifacts prior to image reconstruction. In addition,alternative methods of image reconstruction particularly adapted forreconstructing complex surfaces using complex probes were also desired.

SUMMARY OF THE INVENTION

The preferred embodiment overcomes the drawbacks of slope-based imagereconstruction systems by providing a method of post-filteringre-constructed data points that more easily analyzes and correctsinconsistent image data obtained with a scanning probe microscope tip inan efficient manner that improves the visual rendering of thereconstructed surface and improves both repeatability and accuracy ofthe reconstructed image surface.

According to a preferred embodiment, a method of slope-based imagereconstruction of a re-entrant surface topology employing a scanningprobe microscope is provided. The method generates a slope basedreconstructed image using the data obtained by a tip of the scanningprobe microscope. The data is indicative of, for example, acharacteristic of a re-entrant surface topology of a sample. The step ofgenerating a slope-based reconstructed image includes calculating aslope and an indication of a direction of the slope of the image at aparticular region and determines, using the slope and the indication ofdirection, a probe contact point between the tip and the sample at thatregion. The method further includes the steps of applying a geometricshape filter configured to eliminate reconstructed image data that arenot consistent with a tip shape of the probe used to capture the image;and applying a median filter configured to reduce artifacts in thereconstructed data. The step of applying the median filter is preferablyconducted to reconstruct vectors during extraction vector selectionbased on known tip shape parameters.

Overall, the method of image reconstruction in accordance with thepresent invention enhances a reconstructed image by reducing noise andartifacts associated with noise in the original image data. Thereby,less filtering is required of the original image data and, in somecases, post-processing of the “reconstructed” image data may not benecessary. The invention is also particularly directed to alternativemethods to the slope-base method that provide the capability toreconstruct a re-entrant sample surface.

According to one aspect of the preferred embodiment, a method ofslope-based image reconstruction from data obtained by a tip of ascanning probe microscope is provided. The method generates a slopebased reconstructed image using the data, wherein the data is indicativeof a characteristic of a surface of a sample. The step of generating aslope-based reconstructed image includes calculating a slope of theimage and an indication of direction of the slope at a particular regionand determines, using the slope and indication of slope direction, aprobe contact point between the tip and the sample at that region.Preferably, a smoothing of the reconstructed data is performed.

According to another aspect of this preferred embodiment, the presentinvention provides alternative methods to and of the slope based methodfor performing image reconstruction of data obtained by a tip of asurface scanning microscope. The alternative methods employ algorithmsbased on a principle that no reconstructed image points can physicallyoccupy the same region as the tip during imaging. Sequential translatesof the tip shape or volume sweep out an area or volume that is an“exclusion zone” similar to morphological erosion. The embodiments ofthe alternative methods use either the region defined by the tipboundary or simply the tip boundary.

According to a further alternate embodiment, a method of imagereconstruction of data obtained by a scanning probe microscope (SPM)having a tip includes steps of using the SPM to acquire the data,wherein the data is indicative of a characteristic of a surface of asample. Then, the method includes mapping all points associated theprobe profile translates into a reconstructed image; and applying ageometric type post-filter to exclude all points that fall withinsuccessive translate profiles. A preferred probe includes a first and asecond region. The first region includes an ellipsoidal contact region.The second region includes a supporting rectangular stalk.

In a still further alternate embodiment, a method of imagereconstruction of data obtained by a scanning probe microscope having atip includes the step of acquiring the image data representative of thesample topography with the scanning probe, the image data including anarray of pixel data. In addition, the method includes determining amorphological state of the pixel data by applying morphological tests atthe boundary pixels. The method next includes mapping a first idealizedprobe profile representative of the probe tip at a location of a firstacquired image data, and a second idealized probe profile representativeof the probe tip at a location of a second acquired image data.Thereafter, each pixel data falling within an interior region defined bythe first idealized probe profile is identified and excluded. Lastly,the method includes repeating the above steps for a plurality ofsubsequent idealized probe profiles with respect to the image data so asto generate residual image data representative of the sample topography.

These and other objects, features, and advantages of the invention willbecome apparent to those skilled in the art from the following detaileddescription and the accompanying drawings. It should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are given byway of illustration and not of limitation. Many changes andmodifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred exemplary embodiment of the invention is illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout, and in which:

FIG. 1 is a side elevational schematic view of a prior art probe tipinterfacing with a sample having a complex topography;

FIG. 2 is a side elevational schematic view similar to FIG. 1,illustrating a CD probe tip adapted to image sample surfaces havingcomplex topographies;

FIG. 3A-3C illustrate calibration structures used to characterize a tipof a probe-based instrument, FIG. 3A including a plot of an AFM scan ofa silicon nanoedge;

FIGS. 4A-4C are side elevational schematic views of a sample, an AFMimage profile superimposed thereon, and a reconstructed image profile ofthe sample surface provided by the preferred embodiment, respectively;

FIG. 5 is a side elevational schematic view of tip-sample contact atthree separate positions during a scan, illustrating correction vectorsobtained using the method of the preferred embodiment;

FIG. 6 is a side elevational view schematically illustrating tip-samplecontact at a particular point;

FIG. 7 is a flow diagram illustrating a method of the preferredembodiment;

FIG. 8 is a side elevational schematic view of an image profileillustrating that unit surface normals associated with the sample aredirected away from the interior of the sample;

FIG. 9 is a side elevational schematic view of a dilated image profileusing a tip having non-unique surface normals, illustratingreconstructed points determined according to the preferred embodiment;

FIG. 10 is a flow diagram illustrating an alternate method ofdetermining correction vectors when employing a tip having non-uniquesurface normals, such as that shown in FIG. 2;

FIG. 11 is a side elevational schematic view providing anotherillustration of the method of FIG. 10 when employing a CD tip having aconcave-shaped distal end;

FIG. 12 is a side elevational view of a tip interfacing with a samplesurface at two points;

FIG. 13 is a flow diagram of a method according to the preferredembodiment, whereby two-point contact is identified;

FIG. 14 is a side elevational schematic view illustrating an improvedmethod of computing tip width;

FIG. 14 a is a side elevational schematic view similar to FIG. 3C,illustrating the geometry of the exact tip width equation of thepreferred embodiment;

FIG. 15 is a graph illustrating a comparison of the exact equation, theimproved equation and the prior art equation for performing tip widthcorrection;

FIG. 16 is an orthogonal coordinate system illustrating the anglesassociated with the dimensional correction vector, according to thepreferred embodiment;

FIG. 17 is a schematic diagram of a reconstructed surface topologysuperimposed with noise artifacts illustrating employing a smoothingstep in accordance with a preferred embodiment;

FIG. 18 is a schematic diagram of an idealized probe profile employed asa geometric filter used in a smoothing step of a preferred embodiment;

FIG. 19 is a flow diagram of one embodiment of applying a geometricshape filter;

FIG. 20 is a schematic diagram of an idealized probe profile employed inthe median filter step;

FIG. 21 is a schematic diagram of a window employed in the median filterstep, and useful characteristic dimensions for a CD tip;

FIG. 22 is a schematic diagram of an idealized probe profileillustrating a flyer point;

FIG. 23 is a schematic diagram of an original image topology employing apoint exclusion-based method of image reconstruction in accordance witha preferred embodiment;

FIG. 24 is a schematic reconstructed image topology defined by the pointexclusion method of image reconstruction;

FIG. 25 is flow diagram of a preferred embodiment of the point exclusionbased method of image reconstruction;

FIG. 26 is a schematic diagram of an original image topology employing aprofile tracing method of image reconstruction in accordance with apreferred embodiment;

FIG. 27 illustrates a detailed view of a sequence of idealized probeprofiles employed in the profile tracing method of image reconstructionin accordance with a preferred embodiment;

FIG. 28 is a schematic diagram of a reconstructed image topology definedby the profile tracing method of image reconstruction;

FIG. 29 is a flow diagram of one embodiment of a profile tracing methodof image reconstruction;

FIG. 30 is a schematic diagram of an original image topology employing aprofile tangent method of image reconstruction in accordance with apreferred embodiment;

FIG. 31 is flow diagram of one embodiment of a profile tangent method ofimage reconstruction;

FIG. 32 is schematic diagram of a sequence of translated idealized probeprofiles employed in an intersection method of image reconstruction inaccordance with a preferred embodiment;

FIG. 33 is a flow diagram of a one embodiment of an intersection pointmethod of image reconstruction;

FIG. 34 is a grid of an array of pixilated image data illustrating aboundary erosion method of image reconstruction in accordance with apreferred embodiment;

FIG. 35 is an array of pixilated data representative of an idealizedprobe profile employed in the boundary erosion method in accordance witha preferred embodiment;

FIG. 36 is a sequence of translated idealized probe profilesillustrating the boundary erosion method of image reconstruction; and

FIG. 37 is a flow diagram illustrating one embodiment of the boundaryerosion method of image reconstruction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments are directed toward improved methods ofcorrecting reconstructed image data obtained with a scanning probemicroscope by accurately accounting for probe tip shape inreconstructing the image of the sample surface. More particularly, thepreferred method determines the actual point of contact of the probe tipon the sample surface for several points in a dilated image profile,corrects inconsistent image data with the tip shape used to capture theimage data that improves the visual rendering of the reconstructedsurface and improves both repeatability and accuracy of thereconstructed image surface. The present embodiment also providesalternate methods of image reconstruction.

Image Correction Using Point of Tip Contact Determination

Turning initially to FIGS. 4A-4C, a sample 50 to be scanned by an AFM,as well as the resulting “raw” or dilated image data 54, are shownschematically together with the desired corrected image data 56. Inparticular, sample 50 shown in FIG. 4A includes a trench 52 (FIG. 4 b)produced according to a semiconductor fabrication process and having aparticular height and width whose dimensions are in the nanoscale range.Notably, trench 52 has a relatively complex topography includingvertical sidewall positions and undercut regions.

Sample 50 is supported by on a sample holder of the SPM, so as to allowthe sample to be engaged by an SPM tip (20 in FIG. 2, for example). Asthe SPM tip scans sample 50, image data is acquired and stored forfurther analysis by the user. This data profile 54 is illustrated inFIG. 4B as the dashed lines superimposed on the profile of sample 50shown in FIG. 4A. Notably, the image 54 is dilated from the actualsample topography due to the fact that the shape of the tip is reflectedin the data. The preferred embodiment operates to remove this error sothat the dilation is essentially eliminated and a true image 56 ofsample 50 under test can be obtained, as shown in FIG. 4C. Note that thepreferred embodiment is based on tip sample contact, and that thereconstructed image will be dilated in those regions where no tip-samplecontact occurs, e.g., when a dimension of the probe tip is insufficientto contact a sample having a severely undercut region (FIGS. 1 and 5).

To achieve the reconstructed image shown in FIG. 4C, the preferredembodiment implements a method whereby the geometry associated with thepoint of contact of the probe tip on the sample surface is exploited.Note that the principles of the preferred embodiment are presentedherein using a CD tip. This is being done for illustrative purposesonly, and the invention can be implemented when using probe tips of anyshape.

Turning to FIG. 5, a boot-shaped (CD) probe tip 60 of an AFM is scannedacross a surface 64 of a sample 62. Probe tip 60 includes left and rightprotuberances 66, 68, respectively, that along with bottom surface 70 ofa distal end 72 define the active regions of tip 60. The scan shown wasconducted from upper left to lower right along a trench wall 66, thusobtaining the data represented by the series of triangles. Notably, theacquired data represented by the triangles is the dilated data that mustbe corrected to account for the shape of tip 60.

It is important to note that the dilated data (illustrated by the seriesof triangles) is generated using a fixed reference point for tip 60, forexample, the mid-point “x” of tip 60 at its distal end 72. Importantly,this reference point “x” is typically displaced from the contact pointof tip 60 on the sample surface 64, the contact point itself translatingalong the surface of tip 60 as a scan progresses, as noted previously.As the tip interacts with the sample surface, the reference pointtranslates in the scan direction to generate the dilated imagecorresponding to surface characteristics of the sample.

With continued reference to FIG. 5, probe tip 60 is shown in threepositions as it progresses from left-to-right in the scan direction.Like pairs of letters, for example, a-aa, represent the point of contactbetween probe tip 60 and sample 62, and the corresponding position ofthe selected reference point (i.e., “x”) on the AFM tip used to producethe dilated image profile, respectfully. As such, notably, the distanceand direction (i.e, vector) between the points of each pair (e.g., a-aa,b-bb and c-cc) is the amount by which the AFM image data must becorrected to produce the desired reconstructed image. The preferredembodiment provides this correction, as illustrated by the series ofsquare blocks in FIG. 5.

Two of the correction vectors applied according to the present inventionare shown. When it is at position 60′, tip 60 contacts sample 62 atpoint “b,” and reference point “x” is at “bb.” The preferred embodimentoperates to correct the difference between these two points (i.e., thedilation) by analyzing tip-sample surface normals (described below) toidentify a correction factor, for example, a correction vector V₁,having orthogonal components V_(x1) and V_(z1). Similarly, when tip 60is at position 60″, it contacts sample 62 at point “c,” thus generatingdata image point “cc.” Method 80 (FIG. 7) operates to determine andapply correction vector V₂ to translate point “cc” to “c,” Thusextracting the shape of tip 60 at the contact point “c” from the dilatedimage.

As discussed in further detail below, the vertical portion of thedilated image data, and the corresponding vertical portion 69 of thereconstructed image (i.e., square blocks) at about the undercut region“U” of sample 62 is caused by a shaft 71 of tip 60 contacting surface 64of sample 62 at about point “b” of sample 62. In other words, left side66 of tip 60 does not contact sample 62 at about point 65 of the imagedata. As a result, with shaft 71 contacting the overhang, the. portionof the -undercut region “U” to the left of the vertical line of squareblocks (i.e., corrected data) is “shaded,” due to no tip-sample contactin that region.

Turning to FIG. 6, the geometry of tip-sample interaction during AFMdata acquisition is shown. To illustrate this geometry for anyparticular point of contact between probe tip 74 (schematically showncoupled to shaft 75 of probe 73) and sample 106, sample 106 and probetip 74 can be shown idealized in 2-D cross-section as circles, whereby apoint of contact 78 sits in a tangential plane at the interface of thetwo structures. Importantly, at point of contact 78, the surface normals77, 79 to the tangential plane are equal (and opposite) for sample 106and probe 73, respectively. When utilizing a tip that has a shapedefining surface points characterized by a series of unique surfacenormals (as in FIG. 6), these surface normals can be compared to the AFMimage data to identify the exact point of contact of the tip on thesurface. Note that “equal surface normals” indicates that the normalsextend in the same direction.

More particularly, the tip shape at each point is reflected in the datarecorded by the SPM as the slope and indicated slope direction of thedata at that point. By computing the slope and indicated slope directionof the SPM image data (for example, relative to the scan direction orthe X axis of the X-Z plane) and knowing the scan direction, the imageunit surface normal at point 76 is 77′. At the tip sample contact point,the sample unit surface normal 77 is the same. And, with thisinformation, the point of contact of the tip on the sample can bedetermined. Again, the surface normal 79 associated with the probe tipcontact point will be equal to and opposite of the sample unit surfacenormal 77. By knowing surface normal 79, an appropriate correctionvector (previously computed upon characterization of the probe tip)associated with surface normal 79 can be applied to point 76.

For example, with continued reference to FIG. 6, projecting unit surfacenormal 77 to dilated image point 76 yields sample unit surface normal77′ which can be identified by measuring the angle θ it defines relativeto the scan (X) direction, or about 315°. The probe tip surface normal79 at this point is equal and opposite and thus is about 135° relativeto the scan direction. Because the contact points of the probe tip areinitially characterized and stored (in this case, according to the angletheir corresponding surface normal makes to the scan direction), andcorresponding correction factors have been computed, the appropriatecorrection factors can be readily determined and applied to the dilateddata point. In this case, the point on the tip that has a surface normalthat is 135° relative to the scan direction will be stored as such withan appropriate correction factor, for example (−4 nm, +7 nm) in the X-Zcoordinate system.

As highlighted previously, as the SPM continues to scan the sample, thepoint of contact translates along the tip surface, thus typicallydefining a new tangential plane, and a new surface normal. Because thepresent invention is able to determine this point of tip contact at eachpoint in the scan data (assuming unique surface normals associated withthe active region of the tip), appropriate correction vectors for eachscan point can be determined and a reconstructed image of the samplesurface can be generated. As a result, the dilation error introduced bythe tip shape is essentially eliminated.

In sum, because the tip shape is convolved in the SPM image data and thesurface normal of the contact point of the tip is equal and opposite tothe sample surface normal at that point, the point of contact of the tipon the sample can be determined. This is achieved by computing the localslope of the SPM image data which corresponds to the single point ofcontact of the tip on the sample, and then by identifying acorresponding surface normal. As correction vectors associated with theidentified surface normals are determined using the data correspondingto the point of tip contact on a point-by-point basis, a reconstructedimage having a high degree of accuracy can be achieved in a way that isnot computationally intensive.

In this regard, a method 80 of the preferred embodiment is shown in FIG.7. Initially, in Step 82, the shape of the tip employed ischaracterized. More particularly, the potentially active regions of thesurface of the tip are characterized on a point-by-point basis using anyone of a number of techniques. For example, Step 82 may include scanningthe tip over a calibration structure that allows reconstruction of theprobe tip geometry. One type of calibration structure is an improvedsilicon nanoedge (ISNE) discussed above in conjunction with knowntip-width correction techniques. The ISNE may be placed on a scanningsample holder that is positioned in a chamber of, for example, ascanning probe microscope (SPM) that allows imaging of the samplewithout loading or unloading. The ISNE has a known height (e.g., h=0.8microns±0.2 microns), an estimated radius (e.g., r=7.5 nm±2.5 nm) and avertex angle α which is about 8°±1°.

The resulting image is then used to determine tip width and can be usedto define the shape of the lower tip section (i.e., the active region ofthe probe tip). We discuss the ISNE in further detail below in thesection of description of the preferred embodiments entitled “ImprovedTip Width Correction” in the context of computing the tip width of, forexample, a CD or “boot shaped” probe tip. Calibration structures withlateral protuberances can also be used to further define re-entrantfeatures on the probe itself, such as the undercut region of the CDprobe tip. One such structure with lateral protuberances is the “flaredsilicon ridge” (FSR) which may take on the form of an FSR line (FIG. 3B)or an FSR trench. Yet another alternative for the ISNE is a verticalparallel structure (VPS) which may be used to determine the width of thetip. Typically, using the VPS (FIG. 3C), maximum repeatability in tipwidth measurement is achieved. The structure can be used for periodicchecks while in production to verify the tip width value and thevertical calibration (Z piezo scale factor) of the SPM.

Rather than using calibration structures, a scanning electron microscope(SEM) may be utilized to image the probe to define the geometry of theprobe. However, when imaging down to the nano and angstrom scales, SEMresolution is compromised and may be unsuitable for the applicationscontemplated by the present invention. In still another alternative, thenominal dimensions of the probe tip that are provided by the probemanufacturer may be used to characterize the tip shape in Step 82. Thecharacterization of the tip shape for correction vectors may be based ondiscrete points or use of equations, for example, polynomial ortrigonometric equations that can be applied to correct the dilatedimage.

Independent of the particular implementation of the characterizationStep 82, the next step, Step 84, is to determine surface normalsassociated with several points on the active region of the surface ofthe probe. These surface normals extend orthogonally outwardly from thesurface of the probe tip. Preferably, the surface normals are compiledas corresponding to particular angles θ (FIG. 6) in X-Z space toidentify the characterized points or regions of the tip surface. Forthree dimensional X-Y-Z space, as shown in FIG. 16, θ and a secondangle, φ, are used to define the surface normals, where r is 0 to ∞, θis 0 to 360°, and φ is 0 to 180°.

After the tip shape is characterized, appropriate correction factors aredetermined for each point or region of the active portion of the probetip in Step 86. These correction factors are computed relative to areference point of the probe tip, i.e., the point of the tip used toplot the image data acquired by the AFM. These correction factors may bea convenient ΔX and ΔZ (see FIG. 6) (or ΔX, ΔY, ΔZ in X-Y-Z space) of acorrection vector, or more complex equations associated with thecharacterized point or region. The correction vectors are then compiled,together with their associated surface normals, and stored for readyaccess and application during image reconstruction. For example, thearray of surface normals may be stored in a look-up table along with thecorresponding correction vectors for ready access during imagereconstruction.

Then, in Step 88, a scan of a selected sample with a scanning probemicroscope, such as an AFM, is begun. Note that each of the previoussteps 82-86 are probe specific and are preliminary to the primaryfunction of the preferred embodiment which is to reconstruct dilatedsurface data. In Step 90, method 80 optionally acquires an image profileof the sample surface as the scan is conducted. Of course, this is thedilated data obtained by the AFM, i.e., the uncorrected data.

A “smoothing” step (discussed later) may be implemented between steps 90and 92 of the preferred embodiment (See FIG. 7) to provide a “cleaner”profile and thus facilitate the computations of the remaining steps,including determining slope and curvature.

Next, method 80 computes the slope and slope direction of the imageprofile for a region (e.g., associated with a point) along the profileusing at least two points of the acquired raw data in Step 92. Thisslope may be measured relative to the scan direction (X axis) as ΔZ/ΔXwhen correcting two-dimensional data. Similarly, for three-dimensionaldata, the slope of the tangential plane described previously may bemeasured relative to the XY plane. In Step 94, method 80 determines theunit normal (77 in FIG. 6) to the sample surface for that point based onthe slope and the direction of scanning. Notably, with respect to thedirection of scanning, the sample unit surface normal is directed awayfrom the “interior of the sample,” for example, to the left whenscanning and processing the image profile from left to right. Sampleunit surface normals are illustrated in FIG. 8 for several of the evenlyspaced data points of an AFM profile 108 and testing with the preferredmedian filter (discussed later).

Thereafter, in Step 96, method 80, via an angle θ in a look-up table forexample (2-dimensional; θ and φ (FIG. 16) for a 3-dimensional table),compares the unit normal associated with the dilated data (Step 94) tothe stored surface normals associated with tip characterization. In Step98, method 80 determines an appropriate correction factor associatedwith the unit normal for that point. Again, this determination is madebased on the characterization of the tip shape (Step 82).

Knowing the appropriate correction factor (e.g., vector) for the currentpoint of the image profile, method 80 plots a point of a corrected imageprofile (i.e., reconstructed image) in Step 100. Then, method 80 askswhether all points in the dilated image profile have been considered inStep 102. If not, Steps 92-100 are repeated for at least several pointsin the image profile to build the corrected image profile, i.e., theprofile that more closely resembles the actual sample surface. In Step104, method 80 terminates when all points in the profile have beenconsidered. Alternatively, correction vectors may be determined forseveral points of the image profile and then the points may be assembledinto a corrected image profile for presentation to the user. Notably, asdescribed in further detail below, in the case of tips that havenon-unique surface normals, more than one correction factor may beapplied to a point in the dilated data.

Notably, method 80 is not limited to the use of surface normals (Steps94 and 96) for identification of an appropriate correction factor(vector) based on slope. For instance, the slope of the image data maybe used directly with the knowledge that the left side of the probemakes contact while z is descending in a left to right processing of thedata points and the right hand side of the probe is active when z isascending. This logic is reversed when processing the sequence of datapoints from right to left.

The above-described algorithm is particularly suited for correcting SPMdata when imaging with tips having shapes with surfaces that have uniquesurface normals (i.e., where the normal at each point along the contactor active surface of the tip defines a particular or unique angle withrespect to a selected reference), such as the conventional parabolic AFMtip shown in FIG. 1. However, when imaging with tips that have morecomplex shapes that may have multiple surfaces described by the same(non-unique) surface normal, the point of contact is more difficult todetermine. Therefore, to accommodate tip shapes such as that shown inFIG. 2, for example, the inventor has developed modifications toalgorithm 80.

One example of correcting dilated two-dimensional SPM data when using aprobe tip 110 having non-unique surface normals is illustrated in FIG.9. The surface normals are non-unique because, for instance, all surfacepoints along the bottom surface of tip 110 between active probe tippoints labeled “L” and “R” have the same surface normals extendingorthogonally downwardly from the flat bottom.

In this example, tip 110 having a left side 118 and a right side 120 isshown being scanned over a surface 114 of a sample 112 from left toright. The series of dots, •, represent the dilated “raw” data acquiredby the AFM using the center point 117 of a distal end 116 of the tip asa reference. A dilated image results.

For the dilated image data corresponding to the regions of the samplemarked “A,” “B” and “C,” the dilated image data is correcteddifferently. Using the algorithm described previously, for at leastseveral points in the dilated image represented by the series of dots inthe region of the sample surface marked “A,” the preferred embodimentwill correctly determine the point of tip contact on the sample surface114 (computing slope and determining the unit surface normal) withmethod 80, and thus is able to correct the dilated image data byapplying an appropriate correction factor to that point in the dilatedimage. Again, the appropriate correction vector is the correction vectorcorresponding to the surface normal at the determined point of tipcontact. Notably, for region “A,” the point of tip contact is clearly onthe left hand side of CD tip 110.

At the interface between region “A” and region “B,” there is an abruptchange in the slope of the dilated data. As discussed in further detailbelow, there are several potential causes for this. In this case, it istwo-point contact between the tip and the sample surface at points 122and 124, as shown in FIG. 9. In other words, the width of tip 110 is toobroad to contact the “valley” region 126 at the interface between region“A” and region “B.” As a result, without further enhancement of thereconstructed image (described below, FIG. 12), the shape of the valleyregion of sample surface at the A-B interface is “shaded.”

Notably, at this interface point, tip-sample contact transitions fromleft side 118 of tip 110 to right side 120 of tip 110. Thereforecorrection vectors switch from being directed to the left in region “A”to being directed towards the right in region “B.” This is representedsymbolically in FIG. 9 where the circles represent the corrected datapoints obtained from active probe points on the right side 120 of tip110, while the cross (“X”) marks represent the corrected data pointsobtained from active probe points on the left side 118 of tip 110.

Continuing from left to right, at the interface between the regionsmarked “B” and “C,” a peak 128 in the sample surface 114 is present.When tip traverses peak 128, the first dilated data point is at about130, where probe surface point “R” contacts peak 128. As the scancontinues (left-to-right), tip 110 makes contact with peak 128 atseveral points on the surface of tip between “L” and “R,” each havingthe same downwardly extending orthogonal surface normal. This isreflected in the dilated data (again, dots) between points 130 and 132moving left to right. This flat portion of the dilated image betweenpoints 130 and 132 extends horizontally for a distance approximatelyequal to the distance between “L” and “R” for relatively sharp verticalprotuberance in the sample 114.

Because each of these image points has the same unit surface normal(extending orthogonally upwardly, i.e., corresponding to a tangent thathas a local slope tangent that has a zero slope, the stored “point oftip contact” data relating to that surface normal will not accuratelyidentify the actual point of contact as it moves between points “L” and“R.” As a result, method 80 will not supply a proper correction factorto reconstruct the surface, and will instead generate an image artifact.The present design has taken this into account with an alternate method.

A method 150 according to this alternate preferred embodiment isillustrated graphically in FIG. 9 and described in conjunction with theflow diagram of FIG. 10. Initially, in Step 152, the shape of the tipemployed is characterized using any one of the number of techniquesdescribed previously. Examples of possible techniques to characterizetip shapes are described above in conjunction with FIG. 7 (Steps 82 ofmethod 80). After the shape of tip 110 is characterized, surface normalsare determined in Step 154 and corresponding correction vectors arecomputed in Step 156, a scan of a selected sample with the AFM isconducted in Step 158, as shown and described above in conjunction withFIG. 7. Then, in Step 160, the method generates an image profile of thesample surface. What results is raw or “dilated” data which requirescorrection. Next, as in method of FIG. 7, the method computes the slopeand associated slope direction of the image profile for a first point ofthe profile using at least two points of the dilated data in Step 162.Notably, the slope is measured relative to the X axis (ΔZ/ΔX for thecase of correcting two-dimensional data). Again, as noted previously,for three-dimensional data, the slope of tangential plane is measuredrelative to the X-Y plane.

Thereafter, the method 150 determines the unit surface normal of thesample at the image point being processed in Step 164, similar to method80. In the next step, Step 166, a comparison of the unit surface normalis made with the compiled data corresponding to the surface normals ofthe tip (characterized in Step 152). In the case between data points 130and 132 in FIG. 9, the unit surface normals of the dilated image data inthis region all have infinite slope (i.e., extend orthogonallyupwardly). These unit surface normals could correspond to any one of thesurface normals associated with the points on the flat bottom portion ofdistal end 116 of probe tip 110 (between points “L” and “R”) as each hasinfinite slope (although in an opposite direction, i.e., extendingorthogonally downwardly). Again, because tip 110 has surface points thatdefine non-unique surface normals, method 80 does not provide anappropriate correction factor for the dilated data continuously spanningthe border between “B” and “C” regions shown in FIG. 9.

In this case, the preferred solution is to first determine whether theunit surface normal associated with the dilated data corresponds to aunique surface normal associated with the probe tip (Step 167). If not,in Step 168, two (or more) correction vectors are determined, one foreach of two designated active points on the flat region of tip 110. Inthis case, preferably the end points of the flat region of the probe,i.e., points “L” and “R” shown in FIG. 9. These two correction factorsare then applied to the data point in question in Step 170 and plottedin the generation of a preliminary reconstructed image. In other words,two (or more) reconstructed points are plotted for such data points.However, in the case that the point in the image profile defines asurface normal that is unique to a single point on the sample surface,the method 150 (like method 80) generates an appropriate correctionvector in Step 169 prior to plotting (and/or storing) the corrected datapoint in Step 170.

Next, in Step 172, method 150 determines whether all points in theselected section of the image profile have been analyzed and anappropriate correction vector (or vectors) applied. If not, Steps162-170 are repeated for additional points of the profile. Then, in Step174, method 150 conducts a “minimum filtering” function to accuratelyreconstruct the surface of the sample based on the corrected data.Minimum filtering is employed because it is known that the correctreconstructed point will be the one with the lowest correspondingZ-value for any particular X position, as the sample is always below thetip. More particularly, method 150, in Step 176, analyzes each “X”position in the preliminary reconstructed image, and determines whetherthere is more than one data point associated with that “X” position,approximately, of the preliminary reconstructed image. If so, the methodselects the corrected data point having the smallest vertical or “Z”value, and discards any other point associated with that “X” position.

For the preliminary reconstructed image shown in FIG. 9, where bothcrosses (“x”) and circles (“o”) corresponding to left hand and righthand reconstructed points, respectively, exist, the correctreconstructed point will always be a minimum of those correction pointsat any particular “X” position of the image to more accuratelyapproximate the actual sample surface. As an example, looking at Xposition “X₁” along the slice in FIG. 9, a left hand reconstructed point200 and a right hand reconstructed point 202 are plotted. In this case,left hand reconstructed point 200 is kept, while right handreconstructed point 202 is discarded because point 200 has an associatedZ value that is less than the right hand reconstructed point 202. Inthis fashion, a reconstructed image profile that more closely resemblesthe actual sample topography is generated.

It is important to note that the image data points that cause at leasttwo reconstruction points to be determined in Step 168 are preferablyhighlighted in some fashion so the program understands that these pointsare candidates for the minimum filtering step. Otherwise, if a minimumZ-height filtering step were performed on a scan position of thereconstructed image having two points that, although having different Zheights, are legitimate and should comprise points in the finalreconstructed image, the lesser of the two points in terms of Z heightwould be discarded. For example, this would be the case forreconstructed points 188 (corresponding to an undercut region) and 196in FIG. 12 (described below), which have the same “X” or scan positionbut different Z heights, both legitimate.

Method 150 is not limited to the use of surface normals (Steps 164, 166and 167) for identification of an appropriate correction factor (vector)based on slope. For instance, the slope of the image data may be useddirectly with the knowledge that the left side of the probe makescontact while z is descending in a left to right processing of the datapoints and right hand side of the probe is active when z is ascending.This logic is reversed when processing the sequence of data points fromright to left.

Turning to FIG. 11, in another example of a probe having at least onenon-unique surface normal, a CD probe tip 210 having a concave bottomsurface 212 is utilized to scan a sample. Similar to the flat bottomedCD tip 110 that includes non-unique surface normals, surface normals atpoints Q and P shown in FIG. 11 are non-unique. Again, when implementingthe algorithm of the preferred embodiment, without further modification,method 80 (FIG. 7) is unable to unambiguously determine the point ofcontact on the CD tip when either Q or P contacts the sample surface(such as at 214 or 216 in FIG. 11). Fortunately, the algorithm describedin conjunction with FIG. 10 can be utilized to determine the appropriatecorrection factor. All the appropriate correction factors are computedand plotted as a reconstructed image (“Xs” representing left sidecorrection factors and “Os” representing right side correction factors),including generating two correction factors for dilated image pointsthat have a unit surface normal that is equal and opposite to a surfacenormal associated with more than one point on the surface of the tip 210in Step 168 (for ease of presentation, not all reconstruction points “X”and “O” are shown). Then, the minimum Z corrected point associated witheach scan position of the reconstructed image is selected in Step 174 ofmethod 150. Utilizing this minimum Z height filtering, an accuratereconstructed image can be obtained utilizing even such complex probetip shapes as that shown in FIG. 11.

Next, as mentioned previously in conjunction with FIG. 9 (border regions“A” and “B”), when imaging particular types of features on samples,often times more than one point of a tip 180 having, for example, leftand right boot-shaped sections, 183 and 185, respectively, may contactthe sample surface 184 at the same time, as shown in FIG. 12 at points186 and 188. In the event that such two point contact occurs, the slopeof the acquired data (series of triangles) typically will changeabruptly (at about point 190 of the dilated data). In this case, as thescan continues (downwardly, left-to-right, at point 190), a shaft 181 oftip 180 continues to contact sample surface 184 at point 186, thusyielding the vertical data points shown. Notably, the correspondingreconstructed image (block-shaped points corresponding generally to thesample surface) also goes vertical starting at point 189. It is usefulto identify any such occurrence of two-point contact in attempting toreconstruct the image of the sample surface from the dilated image data.By “flagging” such occurrences, the data can be further analyzed todetermine whether additional portions of the tip can be removed from theimage.

Notably, this abrupt change in slope (2^(nd) derivative of Z withrespect to X) that occurs at image data position 190 (FIG. 12) is anindicator of two (or more) point contact, and can only be created bythis condition if measurement noise is not a factor, and the tip has notbeen damaged. This is due to the fact that the ideal (noiseless) imagecannot measure any slope change greater than the maximum curvature ofthe probe tip. In general, assuming noise is negligible, the anglechange at image data position 190 can serve as either a potentialindicator of two (or more) point contact, or of probe damage that hasresulted in a surface feature with sharp curvature.

To identify such instances of two-point contact, an additional method ofthe preferred embodiment may be implemented accordingly. Thismodification includes analyzing the history of the slopes determined as,for instance, in Step 92 of algorithm 80 (FIG. 7).

Specifically, turning to FIG. 13, upon generating a reconstructed imageaccording to either method 80 (FIG. 7) or 150 (FIG. 10), method 220 isemployed to analyze the resulting data for two-point contact. After astart-up and initialization step, Step 222, the slopes associated withconsecutive points in the reconstructed image are analyzed and a changein slope is or curvature (i.e., ∂²z/∂x²) computed in Step 224. Themethod then compares the change in slope with the maximum change inslope of the probe tip in Step 226. If the image point change in slopeexceeds the maximum value, the point is flagged as an instance ofpossible two-point contact in Step 228. In Step 230, method 220determines whether additional points need to be considered. If so, Steps224-230 are repeated until no points remain. Of course, the algorithm isflexible in that not all data points need to be considered to generate areconstructed image. The method then terminates in Step 232.

One way to determine whether there has been an abrupt change in slope(Step 226) is to determine whether the change in the slope (i.e.,curvature) is greater than the maximum change in slope along the surfaceof the probe tip. The maximum change in slope along the surface of theprobe tip can be determined upon characterizing the tip, for example, inStep 82 of method 80. In FIG. 12, at point 190 the unit surface normaltransitions from approximately 315° relative to the scan or X directionto about 0° relative to the scan direction. Again, this is likely causeddue to shaft 181 and boot-shaped section 183 of CD tip 180 contactingthe sample surface at the same time. As the left-to-right scan continuesin FIG. 12, the shaft continues to interact with the sidewall at aboutpoint 186, and thus the dilated data obtained goes essentially vertical,as one would expect when the vertical shaft is contacting the samplesidewall. In this case, the preferred embodiment may compute twocorrection factors to correct the dilated image point 190 to tworeconstructed points, 188 (the correct reconstructed point), as well aspoint 195, the point (similar to left side of shaft 181, notcharacterized) which has a surface normal that is equal to and oppositethe unit surface normal for vertical image data (i.e., the unit surfacenormal extending in the positive scan direction).

In the preferred embodiment, points 188 and 189 in FIG. 12 are connectedby a straight line to produce the corrected image profile of the samplesurface. However, using an enhancement technique, an image of more ofthe undercut region of the sample can be obtained. In particular,between points 188 and 189 illustrated in FIG. 12, it is known that thearcuate shape of the CD probe tip could have not contacted the undercutregion between these points, and therefore one can provide a betterapproximation of that section of the undercut region by extracting theshape (2-D) or volume (3-D) of the probe between these two known pointson the surface of the probe tip. For instance, the shape of tip 180between points 188 and the point furthest to the left of the left side183 of tip 180 could be extracted between points 188 and 189 of thereconstructed image to produce reconstructed surface 194 shown inphantom.

Improved Tip Width Correction

Turning to FIGS. 14 and 14 a, which are similar to the silicon nanoedgeand its profile shown in FIG. 3A, an improved silicon nanoedge (ISNE) isshown to illustrate how Equations 2 and 3 presented above have beenimproved to more accurately define the tip width of, for example, a CDtip. Again, Equations 2 and 3 presented previously are only accuratewhen the angles (alpha or beta) are essentially zero degrees. Again,because even the improved silicon nanoedge does not have perfectlyvertical sidewalls, this assumption introduces error into thecharacterization of the tip width. An “exact” equation for the endcorrections (W₁ and W₂ which are computed according to Equations 2 and 3discussed previously) is presented below. However, an improved versionof Equations 2 and 3 is first described.

In FIG. 14, the end corrections W₁ and W₂ can be computed with greateraccuracy by accounting for the specification end radii, R_(R) and R_(L)associated with the tip in the computation. The specification and radiiR_(R) and R_(L) are known, as they are provided by the manufacturerand/or are estimated by FSR characterizer data mentioned previously.

Rather than simply multiplying the tangent of the left and right sideslope angles by the height “D” minus the radius of the ISNE (as donepreviously), in this embodiment the specification end radius radii isalso subtracted from the height “D” to improve accuracy of the tip widthcomputation. The new Equations for W₁ and W₂ are defined as follows,W ₁=(D−r−R _(R))tan α+r  Equation 4W ₂=(D−r−R _(L))tan β+r  Equation 5

Although providing an improvement over the prior art tip-width equations2 and 3, an “exact” equation has been developed. Again, the prior artmethod described previously underestimates tip width for alpha greaterthan zero degrees, and thus underestimates CD feature size for a trench,or via, and overestimates CD feature side for a line, ridge, or contact,for example.

Similar to previous embodiments and the prior art, Equation 1 isutilized to determine the width of the tip with image and correctionfeatures (W₁, W₂), but the method of the preferred embodiment accountsfor characteristics of ISNE that were heretofore not considered using anISNE and CD and radii convolution set-up 270 shown in FIG. 14. Inparticular, when employing a CD tip having a known CD tip radius,Equations 2 and 3 (or alternatively, Equations 4 and 5), become,W ₁ =ABS|−cos(α)(r+R _(R))−tan(α)(D−r−R _(R)+sin(α)(r+R _(R)))+R_(R)|  Equation 6W ₂=cos(β)(r+R _(L))−tan(β)(D−r−R _(L)+sin(β)(R+R _(L)))−R_(L)  Equation 7Note that for the case of a vertical sidewall (i.e., β=0 or β=0),equations 6 and 7 degenerate to W₁=r and W₂=r, respectively.

More particularly, with reference to FIG. 14 illustrating the geometryof the above equation as a CD tip (not shown) is scanned over an ISNE272, a first tip contact point 274 (x₁,z₁), at the tangency pointbetween the sidewall and vertex radius, is defined as follows:x ₁ =−r cos α  Equation 8andz₁=r sin α  Equation 9

The point 276 (x₁′, z₁′), which is the position at the base of tip(e.g., “R” in FIG. 9) when at contact point 274, is defined asx ₁′=cos α(r+R _(R)),  Equation 10andz ₁ ′=−R _(R)+sin α(r+R _(R))  Equation 11

Using equations 10 and 11 and solving for x₂′, the “X” position at thebase of tip when the contact point is at a second position 278, W₁ (FIG.3A) can be solved for the right radius of the CD tip, R_(R). W₂ issolved the same way for the left side radius, R_(L).

To reiterate, the dimension “L” is the width of the image at the Zdistance “D” from the plateau of the scanned image, while tip width(W_(tip)) equals L−(W₁+W₂). And, R_(L) and R_(R) are the radii of theleft and right sides of the CD tip (measured from the appropriate tipcenter point, point 275 for right side), respectively, and r is theradius of ISNE 272. The exact equation for the tip-width (Equation 1with end corrections computed as in Equations 6 and 7) yieldssignificantly greater accuracy when performing tip-width correctionanalysis on dilated AFM data, as shown in FIG. 15 which provides agraphical comparison of the exact equation (plot 290), the improvedequation (plot 292), and the prior art equation (plot 294). In thiscase, the ISNE radius is 10 nm, the R_(L) and R_(R) end radii are 30 nm,and the vertical height (D) for measurement is 80 nm. As shown, theprior art equation overestimates the tip-width to be subtracted forangles greater than 0°, and does so to a greater extent than theimproved equation underestimates the tip width.

The scope of the present invention is not limited to the geometry of theISNE (or SNE) structure. For example, characterization of the CD tipwidth could use the sample convolution of the tip radii, characterizerradius (or radii) and sidewall angles in the use of a line or contact,or the upper corners of a trench or via.

In the case of a line, the line may be viewed as an ISNE with anadditional horizontal section which expands the ISNE at the vertex to ahorizontal plateau. The width of the line can be either measured orprovided in standard specifications. The two upper corners of the linemay either have the same or different radii. Similarly, the samerelationships apply to the trench, and the three dimensional analogs ofthe line and trench (namely, the contact and via, respectively).

Smoothing

The smoothing step noted above is particularly effective inreconstructing sample surface topologies having a re-entrant region.FIG. 17 illustrates a sample surface 500 having a re-entrant or undercutregion 505.

Referring to FIG. 17, a series of idealized probe profiles 510, 530,532, 533 and 534 are shown, each representing “a translate” of thescanning probe tip. A translate is an actual or representative profileof the probe tip at a location of acquired data and in a direction ofleft to right scanning of the sample surface 500. The idealized probetip traverses the sample surface 500 as indicated by the triangularsymbols. Each triangular symbol depicts a base 535, 536, 538, 540, and542 of the translated probe profiles 510, 530, 532, 533, and 534 locatedat a X-Z reference along the left to right direction of scanning. Thereconstructed image points of the sample surface topology are shown assquare symbols. The square symbols correspond to reconstructed imagepoints 546, 548, 550, 551, and 552 associated with the translatedidealized probe profiles 510, 530, 532, 533, and 534.

Reference to “idealized” probe profiles can be actual profiles of thescanning probe tip and is not limiting on the invention.

Certain acquired image data represented by the triangular symbols 536,538, and 540, and 542 are superimposed with noise artifacts. FIG. 17illustrates how the superimposed noise can result in erroneousreconstruction of image points 548, 550, and 551, and 552, respectively,relative to the sample surface 500. The noise artifacts can beassociated with actual fluctuations of the probe coordinate systemand/or by instrument noise along the direction of scanning, thusresulting in inconsistent probe and sample surface measurements. Themethod of image reconstruction can employ the slope-method describedpreviously, or the alternate methods of image reconstruction discussedimmediately below.

Geometric Shape Filter

FIG. 19 illustrates one embodiment of a smoothing step 512 to reduce thenoise artifacts depicted in the reconstructed image points 548, 550,551, and 552. Referring to FIGS. 17 and 19, step 527 includes applying ageometric shape filter to remove or exclude the noise artifacts in thereconstructed image data. The preferred step 512 is performed betweensteps 102 and 104 of method 80 illustrated FIG. 7. In contrast, a knowngeometric filter of the prior art is configured to be applied betweensteps 90 and 92 of method 80 (FIG. 7).

The step 527 of applying the geometric shape filter generally includessuperimposing or mapping the translated idealized probe profiles 510,530, 532, 533, and 534 at the location of the acquired data. Thetranslated idealized probe profiles 510, 530, 532, 533, and 534correspond to the acquired original acquired image points withco-location of the probe base 535, 536, 538, 540, and 542 andreconstructed image points 546, 548, 550, 551 and 552.

Referring to FIGS. 17 and 19, step 553 includes determining whether thereconstructed points 546, 548, 550, 551 and 552 fall within otheridealized probe profiles 510, 530, 532, 533 and 534. In general, areconstructed image point falls within another idealized probe profileif it is determined or calculated to be located within the regiondefined by the limits of the geometric shape(s) (discussed below) thatare representative of the scanning probe at “other” locations ofacquiring or measuring the sample surface 500, as illustrated with thebelow example. Step 554 includes excluding reconstructed image points(e.g., reconstructed image points 550 and 552) that fall withingeometric limits representative of other idealized probe profiles 510,530, 532, 533 and 534. For example, FIG. 17 illustrates thatreconstructed image point 550 would be excluded because it is locatedwithin the geometric limits of idealized probe profiles 530 and 533. Inanother example, reconstructed image point 552 would be excluded becauseit is located within the geometric limits of idealized probe profile533. In this way, the smoothing step 512 enhances the reconstructedimage, reduces the problem of noise amplification and artifactgeneration, and eliminates noise induced artifacts without eliminatingcrucial sharp features of the image surface. Moreover, the smoothingstep 512 allows use of the robust slope-based reconstruction techniquesdescribed herein.

FIG. 18 illustrates a preferred embodiment of the idealized probeprofile 510 utilized in applying the geometric shape filter step 527used in the smoothing step 512 described above. Smoothing step 512 canselectively adjust a level associated with the geometric filtering basedon the amplitude of noise in the system. In preferred embodiment of thegeometric shape filter step 527, the amplitude of the root-mean-square(rms) noise is measured based on signal fluctuation with respect to acalculated moving filter average of the amplitude of the acquiredscanning signal. As illustrated in FIG. 18, noise can be measured withrespect to deviation in the amplitude of the signal along the X axis,deviation in the amplitude of the signal along the Z axis, or both. Inparticular, step 512 can define and perform the geometric filtering step527 using an inner region 555 associated with the idealized probeprofile 510. Notably, the inner-region 555 is a selective fraction ofthe size of the idealized probe profile 510, and as such is completelycontained within the idealized probe profile 510. Both the idealizedprobe profile 510 and the inner region 555 can be represented by acombination of convex and/or non-convex regions.

As illustrated in FIG. 18, the idealized probe profile 510 and the innerregion 555 are represented by a combination of geometric shapes thatinclude an ellipsoid (representing the probe tip) and a rectangle(representing the probe stalk 560). Although an ellipsoid shape incombination with a rectangle-shape is shown, the type, number andcombination of geometric shapes can vary to best characterize theidealized profile 510 of the scanning probe based on previous experienceand the complexity of the known shape of the scanning probe.Furthermore, the shape or regions used to represent the inner region mayvary from the shapes/regions used to represent the idealized probeprofile 510.

The fraction of the idealized probe profile that defines the innerregion 555 is a predetermined offset. The predetermined offset can beselectively scaled with respect to noise in the general x-axis directionand noise in the general z-axis direction. Thereby, the extent of thesmoothing step 512 can be directly correlated to the noise in theoriginal image data. The predetermined offset includes an x-offset 565between the inner region 555 and the idealized probe profile 510 in thex-direction. The offset further includes a z-offset 570 between theinner region 555 and the idealized probe profile 510 along thez-direction.

All data points do not need to be tested for inclusion within eachtranslate of the probe tip. Application of the smoothing step 512 can belimited to only candidate original or acquired image data that fallwithin the extreme left and right limits of the inner region 555 in thex-direction and/or the extreme upper and lower limits in thez-direction. Computational savings are obtained by further restrictingthe predetermined offset of the idealized probe profile that defines theinner region without degrading the quality of the noise attenuation andfiltering.

Median Filter

Referring to FIGS. 19-21, the smoothing step 512 can further include astep 580 of applying a median filter to reconstructed vectors 605 of anidealized probe profile 610. Referring to method 80 illustrated in FIG.7, the step 580 of applying a median filter is preferably performedafter step 98, and thereafter step 580 returns to step 100. In contrast,a certain median filter known in the art is configured to be applied tothe acquired image data after step 90 of FIG. 7.

Referring to FIG. 20, the reconstructed vectors 605 are defined by acontact point or matching slope 612 relative to a probe origin 613.Referring to FIG. 21, the step 580 of applying a median filter generallyincludes applying a “window” 615 to the reconstructed vectors of theidealized probe profile. The window 615 is defined by the geometricalparameters of the idealized probe profile 610 known a priori based oncalibration of the probe shape. The reconstruction vectors 605 arefiltered in accordance to the window 615 set by the geometricalparameters of the idealized probe profile 610. A preferred window 615includes a left and a right probe tip vertical edge height (Z_(L) andZ_(R)), a left and right tip overhang (X_(L) and X_(R)), and a tip width(X_(W)). In contrast to the known method of applying the median filterto pre-process the original image data, the median filter step 580 ofthe preferred embodiment is solely applied to the reconstructed vectors605 based on known geometrical parameters of the idealized probe profile610.

FIG. 22 demonstrates how a median filter functions when applied toreconstruction vectors. A sequence of reconstructed vector points (n−2,n−1, n, n+1, n+2) are shown in relation to an idealized probe tipprofile 620. Each reconstructed vector point (n−2, n−1, n, n+1, n+2) hasbeen determined from a local slope in a direction as calculated from theoriginal image data. The “flyer” point (n) is excluded when filteringwith respect to a measured delta X (ΔX), but not with respect to themeasured delta Z (ΔZ). For example, in a first sequence of reconstructedvector points (n−2), (n−1) and (n) tested, the median filter wouldselect point (n−1) based on X or Z position, i.e., ΔX or ΔZ whencompared to window variables in FIG. 21. Similarly, in a next sequenceof reconstructed vector points (n−1), (n) and (n+1), the median filterwould also select the same point (n+1). However, in yet a next sequenceof reconstructed vector points (n), (n+1) and (n+2) tested, the medianfilter reveals testing with respect to ΔX would appropriately excludeflyer point (n) given the sufficiently large ΔX, whereas testing withrespect to ΔZ would select point (n). The median filtering step 580thereby enhances reduction in noise and noise artifacts superimposed inthe reconstructed image. Thereby, less filtering is required of theoriginal image data, less crucial image data is eliminated, and lesspost-processing of the image data may be necessary. FIG. 22 alsoillustrates that tip shape, i.e., variables in FIGS. 20-22, for example,can provide improvements in filter selection.

Point Exclusion Based Image Reconstruction

Referring to FIGS. 23-25, another embodiment of a method 700 of imagereconstruction of a sample surface 705 is herein referred to as “pointexclusion.” In contrast to the smoothing or geometric filtering step 512of slope based method 80 described above, the point exclusion method 700(see FIG. 25) includes mapping (step 707) all original acquired data(represented by triangular symbols 710), which again corresponds to eachbase of a sequence of translated idealized probe profiles 715, 720, and725 obtained in a certain direction of scanning. Notably, FIGS. 23 and24 illustrate a left-to-right direction of scanning, and the base ofeach translated probe profile 715, 720, and 725 includes an X-Zcoordinate origin.

Referring to FIGS. 23 and 25, step 735 includes applying ageometric-type filter, similar to step 527 described above. Next, FIG.23 illustrates application of the point exclusion based method 700. Instep 735, each idealized probe profile 715, 720, and 725 is defined byan ellipsoid contact region. Starting with the first idealized probeprofile 715, the step 740 includes testing to determine, or calculating,the boundaries or limits defining the other idealized probe profiles 720and 725 that are located within an interior region defined by thegeometric shape(s) representative of the first idealized probe profile715. Known methods in the art can be employed to determine or calculatewhat points of idealized profiles 720 and 725 fall within the interiorregions defined by the geometric shapes that are representative of theidealized probe profile 715. In step 742, the points of portions of linesegments of the geometric shape(s) representative of the idealized probeprofiles 720 and 725 that are calculated to fall within (as denoted by“stars”) the interior region defined by the geometric shape(s)representative of the first idealized probe profile 715 are excludedfrom the reconstructed image. The process is repeated for eachsubsequent idealized probe profile 720 and 725. Referring to FIG. 24,the final “reconstructed” image is shown by residual image datarepresented by line 750, including a residual portion of an initialidealized probe profile 755, a residual portion of a last idealizedprobe profile 760. Note, the idealized profiles 755 and 760 alsoincorporate a rectangular region that can be employed to represent thestalk portion extending upward from the scanning probe tip.

The point exclusion method 700 benefits from general geometrical shapesimplifications, decomposition of complex shapes into more elementaryregions and limiting the candidate tip profiles for testing. Limiting ofprofile candidates may be conveniently restricted to those occupying thesame horizontal region along the x-axis, or even a subset thereof basedon tip width. A preferred embodiment of the point exclusion method 742is to first pre-process acquired original image data with a medianfilter, then superimpose idealized probe profiles on the filtered imagedata. The sequential testing of the idealized probe profiles in step 740and exclusion step 742 leaves a residual, bounding surface representedby line 750 which represents the specimen surface topology.

Notably, methods to reconstruct image data using point exclusionconcepts are known, but these known methods are restricted tosingle-valued, pixilated data. In contrast, the point exclusion method700 of the preferred embodiment is generalized to all topologies(including multi-valued data and re-entrant surfaces) by the applicationof data pairs (i.e., [x1, z1], [x2, z2], . . . [x_(n)z_(n)]) for thetwo-dimensional image reconstruction and the data triplets (i.e., [x1,y1, z1], . . . [x_(n)y_(n)z_(n)]) for three-dimensional imagereconstruction. Another benefit of the point exclusion method 700 of thepreferred embodiment is that residual reconstructed image surfacetopology 750 represents the complete specimen topology swept out by thescanning probe. In particular, the method does not leave “image holes”(as they are known in the literature) associated with the slope-methodwhen encountering sudden slope transitions (an example where this occursis typically at the bottom corner of a sharp trench or line feature).Finally, although the point exclusion method 700 tends to self-filter bythe direct application of the geometric shape of the probe tip, thesmoothing step 512 and/or the median filter step 580 described abovecould be performed with method 700.

Profile Tracing

Referring to FIGS. 26-29, yet another embodiment of a method 800 ofimage reconstruction of a sample surface is herein referred to as“profile tracing.” In contrast to the point exclusion method 700described above, the profile tracing method 800 generally includestesting or analyzing image data with respect to only the boundaries orextreme limits (e.g., the line segments of the geometric shapes) ofactive and subsequent translated idealized probe profiles, and not imagedata associated with interior regions of the respective idealized probeprofiles. The profile tracing method 800 generally includes sequentialtesting of the boundaries of translated idealized probe profiles togenerate a residual representative of the reconstructed sample surfacetopology.

Referring to FIGS. 26 and 29, the method 800 generally includes mapping(step 805) the acquired original image data (represented by triangularsymbols 807) with respect to a first idealized probe profile 805, asecond idealized probe profile 810, and a third idealized probe profile811 of the scanning probe at the location of the respective acquiredoriginal image data.

Referring to FIG. 27, a detailed view of the first idealized probeprofile 805, the second idealized probe profile 810, and the thirdidealized probe profile 811 is illustrated. The first idealized probeprofile 805 is “active” in determining an intersection with thesubsequent idealized probe profiles 810 and 811. Referring to FIGS. 27and 29, the method 800 includes a step 812 of determining or calculatingwhere the first idealized probe profile 805 intersects with thesubsequent idealized probe profiles 810 and 811. The methods used instep 812 for calculating the intersections of the line segmentsrepresenting the idealized probe profiles is performed using knownmethods in the art. As shown in FIG. 27, the first idealized probeprofile 805 intersects the second idealized probe profile 810 atintersection 845. The second idealized probe profile 810 intersects thethird idealized probe profile 811 at intersection 850. As a result, theimage points or line segments defining the first idealized probe profile805 up to the intersection 845 is retained (shown by solid line), andthe remainder of the ideal profile 805 (shown by dashed line) isexcluded or removed from the reconstructed image. Notably, non-adjacentidealized probe profiles (e.g., probe profile 811) must be analyzed aswell as the previous second idealized probe profile 810 (adjacent toprofile 805) in determining an intersection with the active idealizedprobe profile 805. Moreover, up to N profiles should be tested in thisfashion, where N only need extend to idealized probe profiles in thehorizontal region that overlaps or intersects with the currently activeidealized probe profile. If multiple sequential idealized probe profilesare found to intersect with the active idealized probe profile, thesubsequent idealized probe profile with the intersection that is closestin sequence relative to the active idealized probe profile is retained.

Still referring to FIGS. 27 and 29, once the intersection 845 iscalculated or determined, step 816 generally includes retaining theidealized probe profile 805 up to the intersection 845 on an activeimage reconstruction profile (see line 856 in FIG. 28), and excluding orremoving the residual points or portions of the line segment of theidealized probe profile 805 are located beyond or subsequent to theintersection 845 relative to the direction of scanning (illustrated byarrow 857). An algorithm is applied such that the residual portion ofthe second idealized probe profile 810 following the intersection 845 isretained as a candidate for determining intersections with subsequentidealized probe profile 811 relative to the direction of scanning 857.

FIG. 28 illustrates generating a final reconstructed image (step 872 inFIG. 29, represented by line 856). The acquired original image data(represented by the triangular symbols 807) is shown superimposed on thereconstructed image 856. The final reconstructed image 856 is calculatedor defined by the residual image points or line segments starting froman initial probe profile 858 and extending until a last or finalidealized probe profile 860. In regard to the initial idealized probeprofile 858, the subsequent portion (shown as a dashed line) of theidealized probe profile 858 following the intersection point 859 isexcluded. Moreover, in regard to the last idealized profile 860associated with a location of a last acquired original image data(illustrated by triangular symbol 863), the preceding portion (shown asdashed line) of the last idealized probe profile 860 up to the lastcalculated intersection 865 is excluded. The reconstructed image 856thus includes all residual image points of the last idealized probeprofile 860 following after, and including, the last intersection point865.

Although computationally intensive, the profile tracing method 800 canbenefit from general geometrical shape simplifications of the idealizedprobe profiles (similar to the smoothing step 512 described previously)and limiting the candidate idealized probe profiles for testing. Asmentioned above, the limiting of the candidate idealized probe profilemay be conveniently restricted to those occupying the same horizontalregion along an x-direction, or a subset of the horizontal region, basedon an idealized probe profile horizontal width. Also, the reconstructedimage generated by the method 800 can be enhanced by initiallypre-processing the acquired original image data with a median filtersimilar to step 580 as describe previously, then superimposing theidealized probe profiles on the filtered original image data andcalculating the intersections as described above. Another embodiment(not shown) of the method 800 includes initially calculating all theintersection points of all of the translated idealized probe profiles inthe sequence, and then proceeding to select an active probe profilebased on the most recent, or lowest, “N” point intersection (in thecurrent active profile) in the sequence and repetitively identifyingintersections with subsequent idealized probe profiles.

In contrast to certain known methods in the art, the method 800 is notrestricted to single-valued, “pixilated data.” Instead, the profiletracing method 800 in accordance with the invention may be generalizedto all surface topologies including multi-valued data and re-entrantsurfaces by the application of data pairs for the two-dimensionalanalysis and data triplets for three-dimensional analysis. The profiletracing method 800 provides a complete specimen surface topology 856represented by the remaining (or residual) reconstructed data swept outby the scanning probe. In addition, the profile tracing method 800 tendsto be self-filtering by the direct application of the geometric shaperepresentative of the idealized probe profile. Finally, although theprofile tracing method 800 tends to self-filter by the directapplication of the geometric shape of the probe tip, the smoothing step512 and/or the median filter step 580 described above could be performedwith method 800.

Profile Tangent Method

FIGS. 30 and 31 illustrate another embodiment of a method 900 of imagereconstruction herein called the profile tangent method. In contrast tothe profile tracing method 800 described above, the profile tangentmethod 900 does not generate a reconstructed image that follows theimage points and/or line segments that comprise the geometric shapesused to define the idealized probe profiles. Rather, the profile tangentmethod 900 generally starts from one valid contact point and then linksup to the next tip translate by identifying a line, anchored from thefirst contact point and just connecting with the subsequent tiptranslate by “touching” it at a first point of tangency. The tangencypoint between the line and tip region (or translate) then serves as ananchor point for the next linked line.

Referring to FIGS. 30 and 31, the profile tangent method 900 includesmapping (step 901 in FIG. 31) original image data (shown as triangularsymbols), and superimposing translated idealized probe profiles relativeto the location of the acquired original image data (step 902 in FIG.31). FIG. 30 illustrates a sequence of idealized probe profiles in aleft to right direction of scanning (illustrated as arrow 903),including a first idealized probe profile 905 and a second idealizedprobe profile 910 in relation to a sample surface 912.

The profile tangent method 900 further includes testing or calculating(step 921 in FIG. 31) a first point of tangency 915 of the first profile905 with the sample surface 912. Step 921 further includes testing orcalculating a line 925 defined between the first tangency point 915 ofthe first idealized probe profile 905 and a point 920 of the secondidealized probe profile 910 where the line 925 is just tangent to theprobe profile 910. Step 922 (see FIG. 31) includes retaining linesegments or image points that define the initial idealized probe profile905 up to (relative to the direction of scanning 903) the first contactpoint 915 and including the tangent line 925 itself. In calculating thetangent lines, known algorithms or methods in the art can be used tocalculate a tangent line between a point and a polygon, and between twopolygons. The second tangent point 920 of the second idealized probeprofile 910 then serves as a starting point in determining (step 927 inFIG. 31) a subsequent tangent line 930 connecting the second tangentpoint 920 to a third tangent point 935 of a third idealized probeprofile 940. The repetitive linking of the tangent points of thesequence of idealized probe profiles with tangent lines in the directionof scanning 903 (step 942 in FIG. 31) thereby generates or provides acomplete, reconstructed image (not shown) of the sample surfacetopology.

The profile tangent method 900 may be less computationally intensivethan other methods of image reconstruction. The profile tangent method900 may produce fewer noise artifacts in the reconstructed image whenprocessing sparse image data (i.e., a situation where the spacing is ofthe same order as the probe tip radius of curvature).

Notably, the profile tangent method 900 can be enhanced by firstpre-filtering the acquired original image data with the median filter,similar to step 580 described previously. Speed enhancements can begained by using geometrical shape simplifications of the idealized probeprofile as described previously. Moreover, the reconstructed image (notshown) of the profile tangent method 900 can be improved by thepost-application of the geometric filter in the smoothing step 512and/or the median filter step 580 described previously.

The profile tangent method 900 is not limited to single-valued,pixilated data as certain known methods known in the art. Rather, theprofile tangent method may be generalized to all surface topologiesincluding multi-valued data and re-entrant surfaces by the applicationof data pairs for two-dimensional analysis and data triplets forthree-dimensional analysis. The profile tangent method 900 is extendableto three-dimensional analysis by substitution of a tangent plane for atangent line in two-dimensional analysis.

Intersection Point Method

Another embodiment of a method 1000 of image reconstruction is hereinreferred to as the “intersection point” method. Referring to FIGS. 32and 33, the intersection point method 1000 includes mapping (step 1001in FIG. 33) the original image data (not shown), and superimposing (step1002 in FIG. 33) the sequence or translations of, for example, a firstidealized probe profile 1005, a second idealized probe profile 1010, athird idealized probe profile 1015, a fourth idealized probe profile1020, and a fifth idealized probe profile 1025 (similar to the step 707described previously).

Step 1026 (see FIG. 33) includes calculating or determining where theidealized probe profiles 1005, 1010, 1015, 1020, and 1025 intersect oneanother. As illustrated in FIG. 32, the first and subsequent secondidealized probe profiles 1005 and 1010 intersect at intersection point1030. The second and subsequent third idealized probe profile 1010 and1015 intersect at intersection point 1035. The third and subsequentfourth idealized probe profiles 1015 and 1020 intersect at intersectionpoint 1040. The fourth and subsequent fifth idealized probe profiles1020 and 1025 intersect at intersection point 1045. Algorithms known inthe art can be used in a known manner to calculate the intersectionsbetween idealized probe profiles 1005, 1010, 1015, 1020, and 1025.

The intersection point method 1000 further includes determining orcalculating (step 1042 in FIG. 33) line segments 1050, 1055, and 1060interconnecting or linking the intersections 1030, 1035, 1040 and 1045,respectively. Only the intersections 1030, 1035, 1040, and 1045 and theline segments 1050, 1055, and 1060 linked therebetween are retained ingenerating or reconstructing a final reconstructed image (not shown) ofthe sample surface topology (step 1062 in FIG. 33).

A preferred embodiment of the intersection point method 1000 can furtherinclude pre-filtering the original data with a median filter, similar tostep 580 described previously. The speed of the method 1000 can beenhanced by application of geometrical shape simplifications torepresent the idealized probe profiles 1005, 1010, 1015, 1020, and 1025.Following application of the certain known algorithms to locate theintersections 1030, 1035, 1040, and 1045 and the connecting linesegments 1050, 1055, and 1060 therebetween, the smoothing step 512and/or the median filter step 580 described previously can be usedfollowing application of the method 1000 to post-filter or removeintersections that would be swept out (e.g., located within the interiorregion) by the actual probe tip as it moves in the direction ofscanning.

In contrast to certain known image reconstruction methods, theintersection point method 1000 is not limited to single-valued pixilateddata as other certain known methods. Rather, the intersection pointmethod 1000 may be generalized to all surface topologies includingmulti-valued data and re-entrant surfaces by the application of datapairs for two-dimensional analysis or data triplets forthree-dimensional analysis.

Boundary Erosion Method

FIGS. 34-37 illustrate still another method 1100 of image constructionin accordance with the present invention referred to as “boundaryerosion.” The boundary erosion method 1100 in general includesconverting re-entrant data pairs and data triplets to a pixel-type array(e.g., array of values illustrated in FIG. 34). The pixel array definedby the original image data (shown as triangular symbols 1103) is thendilated or eroded by applying known algorithms in a known manner withrespect to binary or grayscale morphology to a selected or active pixelwith respect to its neighboring pixels. Of particular interest iserosion because the original surface topology is automatically dilatedby the probe tip shape during scanning. For binary or grayscalemorphology, “erosion” generally involves calculating or determiningwhether to set the value of an active pixel to the minimum value of allpixels in the neighborhood, as defined by an idealized probe profile. Incontrast to certain known methods, the selected or active pixel islimited in range to only the boundary pixels defined by the acquiredoriginal image data.

Referring to FIGS. 34 and 37, the method 1100 includes mapping (step1101 in FIG. 37) the original image data pairs (shown as triangularsymbols 1103) on an X-Z grid 1105. The X-Z grid 1105 includes apixel-type array with assigned values (i.e., 0, 1, and 2) representativeof a morphological state. In a preferred embodiment, each pixel in thearray is initially assigned a value of zero as representative ofbackground state. Reference 1110 illustrates, by way of example,calculating or determining (step 1107 in FIG. 37) a nearest neighboringpixel with respect to each superimposed original image data pair 1103(or data triplets in three-dimensional analysis). The step 1107 ofdetermining or calculating the nearest neighboring pixel can beperformed in a known manner using “nearest neighbor”—determiningalgorithms known in the art. Step 1108 includes assigning and storing avalue of “1” to each nearest matched neighboring pixel 1110. In step1108, the identified nearest neighboring pixel 1110 is stored in aseparate X-Z grid 1112 that will be used to compare with thereconstructed image grid.

Alternatively, the determined nearest neighboring pixel 1110 may beconveniently converted to another value (e.g., “2's”) and used directlyin the active X-Z grid 1105 under analysis. By way of example and inreference to pixel 1115 in FIG. 34, the method 1100 further includesassigning (step 1117 in FIG. 37) a state value of “1” to a pixel 1115 inthe array that is within a “boundary” defined by the pixel state valuesof “2” and in reference to a direction of scanning (illustrated by arrow1118). The pixel state values of “1” thus represent a cross-section ofthe specimen, and the pixel state values of “2” thus represent asurface. In the case of storing a separate grid to represent theboundary, all interior specimen pixels in the active (or“reconstructed”) grid would also be converted to “1”.

Referring to FIG. 35, an idealized probe profile 1120 (shown as a dashedline) is also treated in a similar fashion. Binary values (e.g., “1's”)are used in a known manner to define the active pixels 1125 with respectto pixels assigned with a background state of “0.” The active pixelspreferably include pixels located in an interior region defined by theidealized probe profile 1120. Yet, although the boundary erosion method1100 is illustrated using binary values of 0's and 1's, or otherintegers (e.g., “2” to signify a boundary), the method 1100 is notlimited to these binary numerals, binary images or specific numericalvalues.

In particular, the tip array (referred to as a “structuring element” inimage morphology) has its origin, the pixel shown as 1125 in FIG. 35aligned with the boundary point in the reconstructed image. Referring toFIG. 36, step 1122 (see also FIG. 37) identifies mapping the tipstructuring element to the image boundary point (e.g., point 1125 shownfor the idealized probe profile 1123). Step 1124 includes calculating orsimple numerical comparison between all “active” pixels represented byvalues of “1” in the idealized probe profile 1120 of FIG. 35 and theactive image pixel in the reconstructed image. Those image pixels foundto fall within the boundary limits of the probe profile are re-assignedwith a background state value of “0's” (e.g., pixel 1128 falls withinidealized probe profile 1120 and 1121). These steps are repeated inapplication of the tip structuring element to all image pixels followingalignment of the tip profile or structuring element to original imagepoints.

Step 1126 includes generating a reconstructed specimen surface topology(not shown) as it is defined by the residual “1's” in the pixel array.The pixel array thus defines a reconstructed specimen surface topologyand a cross-section (e.g., represented by “1's) of the specimen. In apreferred embodiment of the method 1110 (i.e., not using a separateboundary array or grid), the reconstructed pixel array uses 2's toindicate boundary points of the acquired surface topology as well as theorigins of the idealized probe profiles 1120, 1121, 1122, 1123, or 1124,the 2's can be converted to 0's at the end of the processing sequence.Finally, if desired, the pixel array can be converted back to a stringof sequenced format data (e.g., data pairs or triplets) representing thespecimen surface topology.

The boundary erosion method 1100 can be enhanced with pre-filtering theoriginal image data with a median filter similar to step 580 describedpreviously. In addition, the speed of the method 1100 can be enhancedwith use of geometric simplifications and resorting to an X-Z array intwo-dimension analysis or X-Y-Z array in three-dimensional analysis. Themethod is easily extensible to three-dimensional analysis with use ofthe three-dimensional image arrays, three-dimensional imagerepresentations of the probe tip shape, and algorithms for nearestneighbor selection in three-dimensional space. The smoothing step 512and/or the median filter step 580 described previously could beperformed with the method 1100 to post-filter or remove intersectionsthat would be swept out (e.g., located within the interior region) bythe actual probe tip as it moves in the direction of scanning.

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the present inventionis not limited thereto. It will be manifest that various additions,modifications and rearrangements of the features of the presentinvention may be made without deviating from the spirit and scope of theunderlying inventive concept.

1. A method of reconstructing a plurality of image data acquired by ascanning probe, the method comprising the steps of: generating an imageusing the plurality of image data, each of the plurality of image databeing indicative of a characteristic of a surface of a sample; mapping aplurality of probe profiles on the image, each of the plurality of probeprofiles being representative of a probe tip at a location of theassociated image data; testing whether each of the image data associatedwith one of the plurality of probe profiles is located within limitsdefined by another of the plurality of probe profiles; using acomputational device for excluding image data that falls within thelimits defined by another of the plurality of probe profiles; andrepeating the above steps for a plurality of subsequent probe profileswith respect to the image data so as to generate residual image datarepresentative of the sample topography.
 2. The method of claim 1,wherein the step of mapping the probe profiles includes generating oneor more geometric shapes that correlate to the limits of the probeprofile.
 3. The method of claim 2, wherein a combination of geometricshapes includes an ellipsoid contact region and a rectangular-shapedstalk region.
 4. The method of claim 2, wherein the testing step isrestricted to testing whether the image data associated with one or moreof the plurality of probe profiles is occupying a common horizontalregion with another of the plurality of probe profiles along an x-axis.5. The method of claim 2, wherein the image data includes anx-coordinate and a z-coordinate.
 6. The method of claim 2, wherein theimage data includes an x-coordinate, a y-coordinate, and a z-coordinate.7. The method of claim 2, further comprising applying a geometric shapefilter to the residual image data.
 8. The method of claim 1, wherein theimage data is pre-filtered with a median filter.
 9. A scanning probemicroscope (SPM) comprising: a probe that interacts with a sample toacquire image data; and a computational device that, maps a plurality ofprobe profiles on the image, each of the plurality of probe profilesbeing representative of a probe tip at a location of the associatedimage data; tests whether each of the image data associated with one ofthe plurality of probe profiles is located within limits defined byanother of the plurality of probe profiles; excludes image data thatfalls within the limits defined by another of the plurality of probeprofiles; and repeats the above steps for a plurality of subsequentprobe profiles with respect to the image data so as to generate residualimage data representative of the sample topography.
 10. A method ofreconstructing a plurality of image data acquired by a scanning probe,the method comprising the steps of: generating an image using theplurality of image data obtained by the scanning probe of a scanningprobe microscope, each of the plurality of image data being indicativeof a characteristic of a surface of a sample; mapping a plurality ofprobe profiles on the image, each of the plurality of probe profilesbeing representative of a probe tip at a location of the associatedimage data; testing whether each of the image data associated with oneof the plurality of probe profiles is located within limits defined byanother of the plurality of probe profiles; using a computational devicefor excluding image data that falls within the limits defined by anotherof the plurality of probe profiles; and repeating the above steps for aplurality of subsequent probe profiles with respect to the image data soas to generate residual image data representative of the sampletopography.
 11. The method of claim 10, wherein the step of mapping theprobe profiles includes generating one or more geometric shapes thatcorrelate to the limits of the probe profile.
 12. The method of claim11, wherein a combination of geometric shapes includes an ellipsoidcontact region and a rectangular-shaped stalk region.
 13. The method ofclaim 11, wherein the testing step is restricted to testing whether theimage data associated with one or more of the plurality of probeprofiles is occupying a common horizontal region with another of theplurality of probe profiles along an x-axis.
 14. The method of claim 11,wherein the image data includes an x-coordinate and a z-coordinate. 15.The method of claim 11, wherein the image data includes an x-coordinate,a y-coordinate, and a z-coordinate.
 16. The method of claim 11, furthercomprising applying a geometric shape filter to the residual image data.17. The method of claim 10, wherein the image data is pre-filtered witha median filter.